Many of the towns, where apartments can sell for over £500,000, lie largely empty and lifeless for most of the year.

Most seaside towns in Northern Ireland have similar architecture, dating from the 19th and early 20th centuries. But these elegant, old homes are being torn down and replaced with blocks of flats.

Properties with development potential are selling so fast that opponents of over-development fear the worst-affected areas will soon lose their character.

Yvonne Hamilton, a Porstewart resident who is opposing over-development, said: "Some second owners seem to be selfish - so long as they have a second home, some don't appear to care if it lies empty while others have no home at all.

"We have lost a generation of first-time buyers while older owners are living in increasing isolation."

The most affected towns in Northern Ireland are, from northwest to southeast, Castlerock, Portstewart, Portrush, Portballintrae, Cushendun, Newcastle and Rostrevor.

Jim Wells, DUP MLA for South Down, who has a reputation as a conservationist, said: "It is almost entirely a coastal phenomenon. Even towns on Lough Erne are not so affected."

Mr Wells added that much of the development was "highly aggressive, and totally unsympathetic to the existing feel of the towns".

Coleraine councillor Christine Alexander said: "It is heartbreaking to see wonderful old Victorian houses, built with great care and craftsmanship, replaced with huge, cheap, tasteless blocks of flats."

In Portrush, 'For Sale' signs, typically accompanied by a line advertising " development potential", are visible everywhere.

"There is not a street in this coastal area where you would not see such a sign," said Ms Alexander.

Portballintrae, where holiday properties can sell for £600,000, no longer has a shop or a post office and has lost two hotels to apartments.

Some of the towns' streets are protected as Areas of Townscape Character, but conservationists say that offers weak protection.

Until recently, permission was not required for a demolition, but now it is. Even so, permission can be granted, leaving the developer free to flatten an old house and apply for apartments in replacement. Even if they demolish without consent, the fines are "derisory" said Mr Wells.

"A £5,000 fine is a joke. Even a £100,000 fine will not scare someone who can make several times that in profit.

"Being forced to rebuild a demolished property exactly is a real deterrent."

Some campaigners see a glimmer of hope as opposition becomes more widespread.

Ms Alexander, for whom planning is the main issue, now tops the poll in her Skerries ward.

Some economic pundits say prices for second properties are so high the bubble may burst.